Pixellated radiation detector structures comprising a detector material having a pixellated structure to define a plurality of individual elements across an area of detector material, typically to convert an area modulated incident radiation into an area modulated photoelectric response, have found increasing application in recent years. The principle is used for instance in silicon or other semiconductor based charge coupled devices (CCDs) for use in digital photography, optical and IR spectroscopy and the like.
For such applications it is typically necessary to bond a detector comprising suitably pixellated semiconductor material both conductively and mechanically to a substrate carrying or connecting with control electronics. In particular, a substrate is an electronic substrate, such as a printed circuit board, or a semiconductor chip such as an integrated circuit etc.
A common technique is referred to as bump bonding or flipchip bonding. In accordance with this technique, an appropriate pattern of bumps of conductive material is first laid down either on the detector structure or on a substrate circuit by any suitable technique, the bump pattern corresponding to the desired conductor connection pattern. A bump may be laid down for example by an evaporation, sputtering or plating technology. Suitable techniques include evaporation through a mask, evaporation with a thick photoresist, screen printing, stud bumping, electroplating and vias, and the provision of conductive polymer bumps.
The material of the bump bond is a material which is such as to establish a conducting connection between the two components when brought together when subject to a secondary treatment. For example it is a reflowable or annealable conductor such as a solder or the like, a curable conductive adhesive etc. To effect this conductive connection with the second element, the first element, onto which the bump has been placed, is flipped and placed into correctly positioned contact with the second element, and the bump bond material is reflowed, cured or otherwise treated to effect a selectively patterned conductive connection between the detector and the substrate. This necessarily leaves gaps in the unconnected regions between the conductive connection pattern. These gaps are underlined with a suitable dielectric material to secure an improved mechanical bond.
It is a characteristic of the flipchip bonding process that the conductive bond is created first, and the mechanical bond conferred by the underfill is created secondarily. Practical considerations often mean that the material forming the conductive bond is a low temperature bonded material, and that the electrical bonds are small. These and other factors can leave them mechanically weak and fragile. Over time, as the bonded structures are subject to mechanical stresses and strains, the conductive bonds can break resulting in a loss of conduction path. The underfill process is intended to give some additional mechanical strength, but is difficult to achieve consistently, especially with complex pixel structures.
Typical pixel detectors based on silicon and like semiconductor materials, for example for use in CCDs, tend to be provided for optical or near-optical range applications with pixel sizes typically less than 200 μm and often less than 100 μm.
Newer materials such as large direct band gap semiconductor materials that are designed to operate with much higher energy radiation, and for example to operate as direct band gap X-ray or gamma-ray pixel detectors, may be less susceptible to such a technique. Both mechanical and dielectric considerations are different. A larger pixel size is typical, for example up to 2 mm. Much higher radiation densities can be expected in use. The material of the device structure has to be relatively thicker. Mechanical considerations can be very difficult when compared with optical or infrared silicon devices. Inconsistent underfill flow can be more of an issue. Silicon technology does not necessarily transfer readily to such materials and pixellated structures.